Fuel failures that occur during routine operation of nuclear reactors can lead to lengthy shutdowns to find the leaking assemblies using current failed-fuel identification means (fuel sipping and eddy-current testing). This can lead to lost revenue and increased radiation exposure for the plant staff during sipping operations. Argonne National Laboratory (ANL) has developed a system called gas tagging for accurate and inexpensive identification of failed fuel assemblies. The gas tagging method consists of the addition of small quantities of enriched noble gas isotopes to each fuel pin in the reactor during pin manufacturing. By systematically varying the compositions of tag gas isotopes from one fuel assembly to another throughout the reactor, each assembly is characterized by it's own unique "tag". If a fuel pin develops a defect during subsequent operation, its tag gas is released to the coolant system along with fission gases that build up inside the pin. Analysis of an off gas sample from the reactor using a mass spectrometer pinpoints the assembly containing the failed pin.
Gas tagging allows failed fuel to be identified during full power operation, thereby saving the utility critical pathway down time, minimizing worker radiation exposure, and enabling core management activities to be initiated in advance of a shutdown. Gas tagging will increase industrial competitiveness for the U.S. nuclear utilities, and hold down electricity costs for consumers and U.S. industrial customers.
Although gas tagging requires only a small quantity of gas for each fuel pin (1 ml or less), the gas itself is expensive and any wastage of gas during fuel pin manufacturing must be minimized. Estimates of wasted gas range from 3 ml to 200 ml per pin, depending upon the fuel vendor. For a wastage of 3 ml, gas tagging is still economically attractive to utility customers. However, for wastage in the range of 200 ml/pin, the cost of tagging would be prohibitive. (Tag gas will cost between $0.10 and $1.25 per ml, and a typical pressurized water reactor core contains 10.sup.5 pins per core).
Current options available to fuel vendors for addressing this problem include (1) modifying their fuel fabrication line to minimize gas wastage; (2) add a laser-puncture unit down stream of the fuel fab line to puncture the pin, add the tag gas, and reseal the puncture; and (3) integrate Argonne National Laboratory's fluid-based technology for recovery of the wasted tag gas. However, none of these options is appropriately viable at this time.